A.M.
First off, I think your son is obviously embarrassed by his speech. That's somewhat normal for kids with artic disorders. Do the exercises your sons SLP recommended, but otherwise drop verbal focus for a bit. Instead, see about speaking softer yourself and as a family. Not mumbling, just softer so your son doesn't feel like odd man out. Also, get him into some type of activity he's good at so he can work on his confidence that way. Make sure you praise him often about other things. If necessary, put lack ofunderstanding on yourself: "my ears must be getting old, can you say that again?". Takes pressure off him. No need for a new therapist unless your son isn't making any progress. Those sounds are palatal and can take a while to master. Plus, therapy takes awhile. Also, when school starts, talk to your SLP and see what other suggestions she has. Sometimes, kids don't mumble for their SLP's because therapy is a safe zone and they fit right in with the other kids. Be patient and positive!